Imaging lens

ABSTRACT

There is provided an imaging lens with high-resolution which satisfies demand of the wide field of view, the low-profileness and the low F-number, and excellently corrects aberrations. An imaging lens comprises in order from an object side to an image side, a first lens having positive refractive power and a convex surface facing the object side near an optical axis, a second lens having negative refractive power near the optical axis, a third lens, a fourth lens, a fifth lens having a concave surface facing the image side near the optical axis, and a sixth lens having a concave surface facing the image side near the optical axis, wherein an image-side surface of the sixth lens is formed as an aspheric surface having at least one off-axial pole point, the fifth lens has positive refractive power near the optical axis, an object-side surface of the sixth lens has a convex surface facing the object side near the optical axis, and predetermined conditional expressions are satisfied.

The present application is based on and claims priority of a Japanese patent application No. 2018-038408 filed on Mar. 5, 2018, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in an imaging device, and more particularly relates to an imaging lens which is built in an increasingly compact and high-performance smartphone and mobile phone, an information terminal such as a PDA (Personal Digital Assistant), a game console, PC and a robot, and moreover, a home appliance with camera function, a monitoring camera and an automobile.

Description of the Related Art

In recent years, it becomes common that camera function is mounted in a home appliance, information terminal equipment, an automobile and public transportation. Demand of products with the camera function is more increased, and development of products is being made accordingly.

The imaging lens mounted in such equipment is required to be compact and have high-resolution performance.

As a conventional imaging lens aiming high performance, for example, the imaging lens disclosed in Patent Document 1 below has been known.

Patent Document 1 (CN106990511A) discloses an imaging lens comprising, in order from an object side, a first lens having a convex surface facing the object side and positive refractive power, a second lens having negative refractive power, a third lens having positive or negative refractive power, a fourth lens having positive or negative refractive power, a fifth lens having positive or negative refractive power, and a sixth lens having positive or negative refractive power.

SUMMARY OF THE INVENTION

However, in lens configurations disclosed in the Patent Document 1, when wide field of view, low-profileness and low F-number are to be realized, it is very difficult to correct aberrations at a peripheral area, and excellent optical performance can not be obtained.

The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an imaging lens with high resolution which satisfies demand of the wide field of view, the low-profileness and the low F-number in well balance and excellently corrects aberrations.

Regarding terms used in the present invention, “a convex surface”, “a concave surface” or “a plane surface” of lens surfaces implies that a shape of the lens surface near an optical axis (paraxial portion). “Refractive power” implies the refractive power near the optical axis. “A pole point” implies an off-axial point on an aspheric surface at which a tangential plane intersects the optical axis perpendicularly. “Total track length” is defined as a distance along the optical axis from an object-side surface of an optical element located closest to the object to an image plane. “The total track length” and “a back focus” is a distance obtained when thickness of an IR cut filter or a cover glass which may be arranged between the imaging lens and the image plane is converted into an air-converted distance.

An imaging lens according to the present invention comprises, in order from an object side to an image side, a first lens having positive refractive power and a convex surface facing the object side near an optical axis, a second lens having negative refractive power near the optical axis, a third lens, a fourth lens, a fifth lens having a concave surface facing the image side near the optical axis, and a sixth lens having a concave surface facing the image side near the optical axis, wherein the image-side surface of the sixth lens is formed as an aspheric surface having at least one off-axial pole point.

According to the imaging lens having the above-described configuration, the first lens achieves wide field of view and low-profileness by strengthening the refractive power. The second lens properly corrects spherical aberration and chromatic aberration occurring at the first lens.

The third lens properly corrects the spherical aberration, coma aberration and distortion. The fourth lens properly corrects the spherical aberration, the coma aberration, astigmatism and the chromatic aberration. The fifth lens properly corrects the astigmatism by having the concave surface facing the image side near the optical axis. The sixth lens properly corrects the astigmatism, field curvature and the distortion. An image-side surface of the sixth lens has the concave surface facing the image side near the optical axis, and the field curvature and the distortion can be properly corrected and light ray incident angle to an image sensor can be properly controlled by forming the image-side surface of the sixth lens as the aspheric surface having at least one off-axial pole point.

According to the imaging lens having the above-described configuration, it is preferable that an image-side surface of the third lens has a convex surface facing the image side near the optical axis.

When the image-side surface of the third lens has the convex surface facing the image side near the optical axis, light ray incident angle to the image-side surface of the third lens can be properly controlled and the spherical aberration, the coma aberration and the distortion can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that the fourth lens has negative refractive power near the optical axis.

When the refractive power of the fourth lens is negative, the chromatic aberration can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that an image-side surface of the fourth lens has a convex surface facing the image side near the optical axis.

When the image-side surface of the fourth lens has the convex surface facing the image side near the optical axis, light ray incident angle to the image-side surface of the fourth lens can be properly controlled and the spherical aberration, the astigmatism, the coma aberration and the distortion can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that the fifth lens has positive refractive power near the optical axis.

When the fifth lens has the positive refractive power, it is advantageous to low-profileness.

According to the imaging lens having the above-described configuration, it is preferable that an object-side surface of the fifth lens has a convex surface facing the object side near the optical axis.

When the object-side surface of the fifth lens has the convex surface facing the object side near the optical axis, the coma aberration can be properly corrected.

Furthermore, according to the imaging lens having the above-described configuration, it is preferable that an object-side surface of the fifth lens is formed as an aspheric surface having at least one off-axial pole point.

When the object-side surface of the fifth lens is formed as the aspheric surface having at least one off-axial pole point, the field curvature and the distortion can be properly corrected and the light ray incident angle to the image sensor can be properly controlled.

According to the imaging lens having the above-described configuration, it is preferable that an image-side surface of the fifth lens is formed as an aspheric surface having at least one off-axial pole point.

When the image-side surface of the fifth lens is formed as the aspheric surface having at least one off-axial pole point, the field curvature and the distortion can be properly corrected and the light ray incident angle to the image sensor can be properly controlled.

According to the imaging lens having the above-described configuration, it is preferable that an object-side surface of the sixth lens has a convex surface facing the object side near the optical axis.

When the object-side surface of the sixth lens has the convex surface facing the object side near the optical axis, the astigmatism and the field curvature can be properly corrected.

Furthermore, according to the imaging lens having the above-described configuration, it is preferable that an object-side surface of the sixth lens is formed as an aspheric surface having at least one off-axial pole point.

When the object-side surface of the sixth lens is formed as the aspheric surface having at least one off-axial pole point, the field curvature and the distortion can be properly corrected and the light ray incident angle to the image sensor can be properly controlled.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (1) is satisfied: 5.0<(D3/|f3|)×100<14.5  (1) where D3: thickness along the optical axis of the third lens, f3: focal length of the third lens.

The conditional expression (1) defines an appropriate range of the thickness along the optical axis of the third lens. When a value is below the upper limit of the conditional expression (1), the thickness along the optical axis of the third lens is suppressed from being too small, and formability of the lens becomes excellent. On the other hand, when the value is above the lower limit of the conditional expression (1), the thickness along the optical axis of the third lens is suppressed from being too large, and an air gap of the object side and the image side of the third lens can be easily secured. As a result, the low-profileness can be maintained.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (2) is satisfied: 0.02<T1/T2<0.12  (2) where T1: distance along the optical axis from the image-side surface of the first lens to the object-side surface of the second lens, T2: distance along the optical axis from the image-side surface of the second lens to the object-side surface of the third lens.

The conditional expression (2) defines an appropriate range of a distance between the first lens and the second lens and a distance between the second lens and the third lens. By satisfying the conditional expression (2), difference of the distance between the first lens and the second lens and the distance between the second lens and the third lens is suppressed from being large, and the low-profileness is achieved. Furthermore, by satisfying the conditional expression (2), the second lens is arranged at an optimum position, and aberration correction function of the lens becomes more effective.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (3) is satisfied: 11.5<(T2/f)×100  (3) where T2: distance along the optical axis from the image-side surface of the second lens to the object-side surface of the third lens, f: focal length of the overall optical system of the imaging lens.

The conditional expression (3) defines an appropriate range of a distance along the optical axis between the second lens and the third lens. By satisfying the conditional expression (3), the coma aberration and the astigmatism can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (4) is satisfied: −8.7<f2/f<−2.0  (4) where f2: focal length of the second lens, f: focal length of the overall optical system of the imaging lens.

The conditional expression (4) defines an appropriate range of refractive power of the second lens. When a value is below the upper limit of the conditional expression (4), the negative refractive power of the second lens becomes appropriate and the low-profileness can be realized. On the other hand, when the value is above the lower limit of the conditional expression (4), the chromatic aberration and the spherical aberration can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (5) is satisfied: 0.5<vd5/vd6<1.5  (5) where vd5: abbe number at d-ray of the fifth lens, vd6: abbe number at d-ray of the sixth lens.

The conditional expression (5) defines an appropriate range of each abbe number at d-ray of the fifth lens and the sixth lens. By satisfying the conditional expression (5), the chromatic aberration can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (6) is satisfied: −3.60<(D2/f2)×100<−0.55  (6) where D2: thickness along the optical axis of the second lens, f2: focal length of the second lens.

The conditional expression (6) defines an appropriate range of the thickness along the optical axis of the second lens. When a value is below the upper limit of the conditional expression (6), the thickness along the optical axis of the second lens is prevented from being too small, and formability of the lens becomes excellent. On the other hand, when the value is above the lower limit of the conditional expression (6), the thickness along the optical axis of the second lens is prevented from being too large, and an air gaps of the object side and the image side of the second lens can be easily secured. As a result, the low-profileness can be maintained.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (7) is satisfied: 0.15<(T4/f)×100<1.40  (7) where T4: distance along the optical axis from the image-side surface of the fourth lens to the object-side surface of the fifth lens, f: focal length of the overall optical system of the imaging lens.

The conditional expression (7) defines an appropriate range of a distance along the optical axis from the image-side surface of the fourth lens to the object-side surface of the fifth lens. By satisfying the conditional expression (7), the total track length can be shortened, and the astigmatism and the distortion can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (8) is satisfied: −3.20<f4/f<−0.65  (8) where f4: focal length of the fourth lens, f: focal length of the overall optical system of the imaging lens.

The conditional expression (8) defines an appropriate range of the refractive power of the fourth lens. When a value is below the upper limit of the conditional expression (8), the negative refractive power of the fourth lens becomes appropriate, and the low-profileness can be achieved. On the other hand, when the value is above the lower limit of the conditional expression (8), the chromatic aberration and the spherical aberration can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (9) is satisfied: 0.6<f5/f<2.2  (9) where f5: focal length of the fifth lens, f: focal length of the overall optical system of the imaging lens.

The conditional expression (9) defines an appropriate range of the refractive power of the fifth lens. When a value is below the upper limit of the conditional expression (9), the positive refractive power of the fifth lens becomes appropriate, and the low-profileness can be achieved. On the other hand, when the value is above the lower limit of the conditional expression (9), the spherical aberration, the astigmatism and the distortion can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that composite refractive power of the fifth lens and the sixth lens is positive, and more preferable that a below conditional expression (10) is satisfied: 0.45<f56/f<4.25  (10) where f56: composite focal length of the fifth lens and the sixth lens, f: focal length of the overall optical system of the imaging lens.

When the composite refractive power of the fifth lens and the sixth lens is positive, it is advantageous to the low-profileness. Furthermore, the conditional expression (10) defines the composite refractive power of the fifth lens and the sixth lens. When a value is below the upper limit of the conditional expression (10), the positive composite refractive power of the fifth lens and the sixth lens become appropriate, and the low-profileness can be achieved. On the other hand, when the value is above the lower limit of the conditional expression (10), the spherical aberration, the astigmatism, the field curvature and the distortion can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (11) is satisfied: −6.0<f2/f5<−1.6  (11) where f2: focal length of the second lens, f5: focal length of the fifth lens.

The conditional expression (11) defines an appropriate range of the refractive power of the second lens and the fifth lens. When a value is below the upper limit of the conditional expression (11), the astigmatism and distortion can be properly corrected. On the other hand, when the value is above the lower limit of the conditional expression (11), the negative refractive power of the second lens becomes appropriate and the chromatic aberration can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (12) is satisfied: 0.25<|r7|/f<0.85  (12) r7: paraxial curvature radius of the object-side surface of the fourth lens, f: focal length of the overall optical system of the imaging lens.

The conditional expression (12) defines an appropriate range of the paraxial curvature radius of the object-side surface of the fourth lens. When the value is below the upper limit of the conditional expression (12), the coma aberration can properly corrected. On the other hand, when the value is above the lower limit of the conditional expression (12), the spherical aberration and the distortion can be properly corrected.

According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (13) is satisfied: 1.45<|f3|/f<5.10  (13) where f3: focal length of the third lens, f: focal length of the overall optical system of the imaging lens.

The conditional expression (13) defines an appropriate range of the refractive power of the third lens. When a value is below the upper limit of the conditional expression (13), the chromatic aberration can be properly corrected. On the other hand, when the value is above the lower limit of the conditional expression (13), the spherical aberration, the coma aberration and the distortion can be properly corrected.

Effect of Invention

According to the present invention, there can be provided an imaging lens with high resolution which satisfies demand of the wide field of view, the low-profileness and the low F-number in well balance, and properly corrects aberrations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a general configuration of an imaging lens in Example 1 according to the present invention;

FIG. 2 shows spherical aberration, astigmatism, and distortion of the imaging lens in Example 1 according to the present invention;

FIG. 3 is a schematic view showing the general configuration of an imaging lens in Example 2 according to the present invention;

FIG. 4 shows spherical aberration, astigmatism, and distortion of the imaging lens in Example 2 according to the present invention;

FIG. 5 is a schematic view showing the general configuration of an imaging lens in Example 3 according to the present invention;

FIG. 6 shows spherical aberration, astigmatism, and distortion of the imaging lens in Example 3 according to the present invention;

FIG. 7 is a schematic view showing the general configuration of an imaging lens in Example 4 according to the present invention;

FIG. 8 shows spherical aberration, astigmatism, and distortion of the imaging lens in Example 4 according to the present invention.

FIG. 9 is a schematic view showing a general configuration of an imaging lens in Example 5 according to the present invention;

FIG. 10 shows spherical aberration, astigmatism, and distortion of the imaging lens in Example 5 according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, the preferred embodiments of the present invention will be described in detail referring to the accompanying drawings.

FIGS. 1, 3, 5, 7 and 9 are schematic views of the imaging lenses in Examples 1 to 5 according to the embodiments of the present invention, respectively.

As shown in FIG. 1, the imaging lens according to the present embodiment comprises, in order from an object side to an image side, a first lens L1 having positive refractive power and a convex surface facing the object side near an optical axis X, a second lens L2 having negative refractive power near the optical axis X, a third lens L3, a fourth lens L4, a fifth lens L5 having a concave surface facing the image side near the optical axis X, and a sixth lens L6 having a concave surface facing the image side near the optical axis X, wherein an image-side surface of the sixth lens L6 is formed as an aspheric surface having at least one off-axial pole point.

A filter IR such as an IR cut filter and a cover glass are arranged between the sixth lens L6 and an image plane IMG (namely, the image plane of an image sensor). The filter IR is omissible.

By arranging an aperture stop ST on the object side of the first lens L1, correction of aberrations and control of an incident angle of the light ray of high image height to the image sensor become facilitated.

The first lens L1 has a meniscus shape having a convex surface facing the object side and a concave surface facing the image side near the optical axis X. Therefore, spherical aberration, astigmatism and distortion are properly corrected. A shape of the first lens L1 may be a bi-convex shape having the convex surfaces facing the object and image sides near the optical axis X as in the Example 4 shown in FIG. 7. In this case, the positive refractive power on both sides is advantageous to the low-profileness.

The second lens L2 has a meniscus shape having a convex surface facing the object side and a concave surface facing the image side near the optical axis X. Therefore, the spherical aberration, the astigmatism, field curvature and the distortion are properly corrected.

The third lens L3 has the positive refractive power, and has a bi-convex shape having convex surfaces facing the object and image sides near the optical axis X. Therefore, the spherical aberration, coma aberration and the distortion are properly corrected. A shape of the third lens L3 may be a meniscus shape having a concave surface facing the object side and the convex surface facing the image side near the optical axis X as in the Example 5 shown in FIG. 9. In this case, an incident angle of the light ray to the third lens L3 becomes appropriate and the spherical aberration, the field curvature and the distortion are properly corrected.

The fourth lens L4 has a meniscus shape having a concave surface facing the object side and a convex surface facing the image side near the optical axis X. Therefore, an incident angle of the light ray to the fourth lens L4 becomes appropriate and the spherical aberration, the astigmatism, the coma aberration and the distortion are properly corrected.

The fifth lens L5 has the positive refractive power, and the astigmatism, the field curvature and the distortion are properly corrected while maintaining the low-profileness. A shape of the fifth lens L5 is a meniscus shape having a convex surface facing the object side and a concave surface facing the image side near the optical axis X. Therefore, the coma aberration and the astigmatism are properly corrected.

Furthermore, an object-side surface and an image-side surface of the fifth lens L5 are formed as an aspheric surface having at least one off-axial pole point, and the field curvature and the distortion are properly corrected and an incident angle of the light ray to an image sensor is appropriately controlled.

The sixth lens L6 has a meniscus shape having a convex surface facing the object side and a concave surface facing the image side near the optical axis X. Therefore, the astigmatism, the field curvature and the distortion are properly corrected. The sixth lens L6 may have positive refractive power as in the Example 3 shown in FIG. 5. In this case, the low-profileness becomes more facilitated.

An object-side surface and an image-side surface of the sixth lens L6 are formed as an aspheric surface having at least one off-axial pole point, and the field curvature and the distortion are properly corrected and the incident angle of the light ray to an image sensor is appropriately controlled.

Regarding the imaging lens according to the present embodiments, it is preferable that all lenses from the first lens L1 to the sixth lens L6 are single lens. Configuration consisting of only the single lenses can frequently use the aspheric surfaces. In the present embodiments, all lens-surfaces are formed as the appropriate aspherical surface, and proper aberration correction is made. In comparison with the case in which a cemented lens is used, workload is reduced, and manufacturing in low cost becomes available.

Furthermore, the imaging lens according to the present embodiments makes manufacturing facilitated by using plastic material for all lenses, and mass production in a low cost can be realized.

The material applied to the lens is not limited to the plastic material. By using glass material, further high performance may be aimed. It is preferable that all of lens-surfaces are formed as the aspheric surface, however, spherical surfaces easy to be manufactured may be adopted in accordance with required performance.

The imaging lens according to the present embodiments shows preferable effect by satisfying the below conditional expressions (1) to (13). 5.0<(D3/|f3|)×100<14.5  (1) 0.02<T1/T2<0.12  (2) 11.5<(T2/f)×100  (3) −8.7<f2/f<−2.0  (4) 0.5<vd5/vd6<1.5  (5) −3.60<(D2/f2)×100<−0.55  (6) 0.15<(T4/f)×100<1.40  (7) −3.20<f4/f<−0.65  (8) 0.6<f5/f<2.2  (9) 0.45<f56/f<4.25  (10) −6.0<f2/f5<−1.6  (11) 0.25<|r7|/f<0.85  (12) 1.45<|f3|/f<5.10  (13) where vd5: abbe number at d-ray of the fifth lens L5, vd6: abbe number at d-ray of the sixth lens L6, D2: thickness along the optical axis X of the second lens L2, D3: thickness along the optical axis X of the third lens L3, T1: distance along the optical axis X from the image-side surface of the first lens L1 to the object-side surface of the second lens L2, T2: distance along the optical axis X from the image-side surface of the second lens L2 to the object-side surface of the third lens L3, T4: distance along the optical axis X from the image-side surface of the fourth lens L4 to the object-side surface of the fifth lens L5, f: focal length of the overall optical system of the imaging lens, f2: focal length of the second lens L2, f3: focal length of the third lens L3, f4: focal length of the fourth lens L4, f5: focal length of the fifth lens L5, f56: composite focal length of the fifth lens L5 and the sixth lens L6, r7: paraxial curvature radius of the object-side surface of the fourth lens L4.

It is not necessary to satisfy the above all conditional expressions, and by satisfying the conditional expression individually, operational advantage corresponding to each conditional expression can be obtained.

The imaging lens according to the present embodiments shows further preferable effect by satisfying the below conditional expressions (1a) to (13a). 6<(D3/|f3|)×100<14  (1a) 0.025<T1/T2<0.110  (2a) 12<(T2/f)×100<30  (3a) −7.2<f2/f<−2.5  (4a) 0.75<vd5/vd6<1.25  (5a) −3.00<(D2/f2)×100<−0.85  (6a) 0.25<(T4/f)×100<1.15  (7a) −2.65<f4/f<−1.00  (8a) 0.9<f5/f<1.8  (9a) 0.75<f56/f<3.55  (10a) −5.00<f2/f5<−1.85  (11a) 0.40<|r7|/f<0.70  (12a) 1.65<|f3|/f<4.25  (13a)

The signs in the above conditional expressions have the same meanings as those in the paragraph before the preceding paragraph.

In this embodiment, the aspheric shapes of the surfaces of the aspheric lens are expressed by Equation 1, where Z denotes an axis in the optical axis direction, H denotes a height perpendicular to the optical axis, R denotes a paraxial curvature radius, k denotes a conic constant, and A4, A6, A8, A10, A12, A14 and A16 denote aspheric surface coefficients.

$\begin{matrix} {Z = {\frac{\frac{H^{2}}{R}}{1 + \sqrt{1 - {\left( {k + 1} \right)\frac{H^{2}}{R^{2}}}}} + {A_{4}H^{4}} + {A_{6}H^{6}} + {A_{8}H^{8}} + {A_{10}H^{10}} + {A_{12}H^{12}} + {A_{14}H^{14}} + {A_{16}H^{16}}}} & {{Equation}\mspace{14mu} 1} \end{matrix}$

Next, examples of the imaging lens according to this embodiment will be explained. In each example, f denotes the focal length of the overall optical system of the imaging lens, Fno denotes an F-number, ω denotes a half field of view, ih denotes a maximum image height, and TTL denotes a total track length. Additionally, i denotes surface number counted from the object side, r denotes a curvature radius, d denotes the distance of lenses along the optical axis (surface distance), Nd denotes a refractive index at d-ray (reference wavelength), and vd denotes an abbe number at d-ray. As for aspheric surfaces, an asterisk (*) is added after surface number i.

Example 1

The basic lens data is shown below in Table 1.

TABLE 1 Example 1 Unit mm f = 3.24 i h = 2.90 Fno = 1.45 TTL = 4.58 ω(°) = 41.4 Surface Data Surface Curvature Surface Refractive Abbe Number i Radius r Distance d Index Nd Number νd (Object) Infinity Infinity 1(Stop) Infinity −0.3600 2* 2.0256 0.5030 1.544 55.57 (νd1) 3* 16.6496 0.0200 4* 2.0445 0.2200 1.661 20.37 (νd2) 5* 1.5074 0.4120 6* 8.1757 0.8110 1.535 55.66 (νd3) 7* −5.1446 0.2050 8* −1.7899 0.3210 1.661 20.37 (νd4) 9* −3.1664 0.0300 10* 1.9115 0.5820 1.535 55.66 (νd5) 11* 7.9843 0.3820 12* 1.3351 0.3760 1.535 55.66 (νd6) 13* 0.8311 0.3500 14  Infinity 0.2100 1.517 64.20 15  Infinity 0.2273 Image Plane Infinity Constituent Lens Data Lens Start Surface Focal Length Composite Focal Length 1 2 4.190 f56 9.177 2 4 −10.375 3 6 6.032 4 8 −6.869 5 10 4.547 6 12 −5.562 Aspheric Surface Data Second Surface Third Surface Fourth Surface Fifth Surface Sixth Surface Seventh Surface k 5.233217E−01 0.000000E+00 −9.411926E+00 −9.811593E+00 −4.100811E+01 1.342633E+01 A4 8.158016E−03 −1.186416E−01 −1.450062E−01 1.394336E−01 −4.358539E−02 −1.250347E−01 A6 −2.680143E−02 3.764648E−01 3.344519E−01 −3.395908E−01 −4.043112E−02 8.279887E−02 A8 5.137872E−02 −5.503932E−01 −4.295233E−01 5.684687E−01 4.501640E−02 −8.327833E−02 A10 −4.552239E−02 4.653073E−01 3.207126E−01 −5.310654E−01 −6.410140E−02 −1.008230E−01 A12 1.736895E−02 −2.085680E−01 −1.250630E−01 2.485474E−01 2.967330E−02 2.374766E−01 A14 0.000000E+00 3.846520E−02 1.386252E−02 −4.444037E−02 0.000000E+00 −1.467130E−01 A16 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 2.998000E−02 Eighth Surface Ninth Surface Tenth Surface Eleventh Surface Twelfth Surface Thirteenth Surface k 0.000000E+00 −9.736357E+00 −2.568235E+00 1.587690E+01 −4.473342E+00 −3.159869E+00 A4 4.363393E−03 −1.111305E−01 −6.345976E−02 2.918876E−02 −3.986822E−01 −2.328529E−01 A6 2.802829E−01 2.236093E−01 7.572655E−02 −6.743946E−03 2.119517E−01 1.607361E−01 A8 −6.230058E−01 −2.998281E−01 −9.471745E−02 −2.434960E−02 −8.228684E−02 −7.667786E−02 A10 6.042873E−01 2.238453E−01 5.074949E−02 9.529753E−03 2.663079E−02 2.299421E−02 A12 −2.629289E−01 −8.199681E−02 −1.527542E−02 −1.417569E−03 −6.373961E−03 −4.000615E−03 A14 4.184620E−02 1.148990E−02 2.053298E−03 6.655180E−05 9.217919E−04 3.645826E−04 A16 −2.929831E−04 4.832112E−05 −2.607511E−05 1.434388E−07 −5.835510E−05 −1.341432E−05

The imaging lens in Example 1 satisfies conditional expressions (1) to (13) as shown in Table 6.

FIG. 2 shows spherical aberration (mm), astigmatism (mm), and distortion (%) of the imaging lens in Example 1. The spherical aberration diagram shows the amount of aberration at each wavelength of F-ray (486 nm), d-ray (588 nm), and C-ray (656 nm). The astigmatism diagram shows the amount of aberration at d-ray on a sagittal image surface S (solid line) and on tangential image surface T (broken line), respectively (same as FIGS. 4, 6, 8 and 10). As shown in FIG. 2, each aberration is corrected excellently.

Example 2

The basic lens data is shown below in Table 2.

TABLE 2 Example 2 Unit mm f = 3.24 i h = 2.90 Fno = 1.45 TTL = 4.55 ω(°) = 41.1 Surface Data Surface Curvature Surface Refractive Abbe Number i Radius r Distance d Index Nd Number νd (Object) Infinity Infinity 1(Stop) Infinity −0.3500 2* 2.0665 0.5664 1.544 55.57 (νd1) 3* 12.5263 0.0400 4* 2.0629 0.2000 1.661 20.37 (νd2) 5* 1.5674 0.4166 6* 9.2467 0.6333 1.535 55.66 (νd3) 7* −5.6982 0.2313 8* −1.7815 0.3211 1.661 20.37 (νd4) 9* −3.3820 0.0109 10* 1.8023 0.4391 1.535 55.66 (νd5) 11* 6.8229 0.4400 12* 1.1556 0.4275 1.535 55.66 (νd6) 13* 0.8007 0.3600 14  Infinity 0.2100 1.517 64.20 15  Infinity 0.3259 Image Plane Infinity Constituent Lens Data Lens Start Surface Focal Length Composite Focal Length 1 2 4.466 f56 5.892 2 4 −11.764 3 6 6.691 4 8 −6.191 5 10 4.444 6 12 −8.400 Aspheric Surface Data Second Surface Third Surface Fourth Surface Fifth Surface Sixth Surface Seventh Surface k 5.713411E−01 0.000000E+00 −9.850000E+00 −1.121782E+00 −7.769580E+01 1.677788E+01 A4 5.579520E−03 −1.181312E−01 −1.539512E−01 1.367737E−01 −4.414767E−02 −1.207886E−01 A6 −2.329773E−02 3.562168E−01 3.394534E−01 −3.356960E−01 −4.650742E−02 7.552949E−02 A8 4.522983E−02 −5.368933E−01 −4.842085E−01 5.588351E−01 6.526604E−02 −8.354781E−02 A10 −3.999135E−02 4.797823E−01 4.056339E−01 −5.356032E−01 −7.297754E−02 −1.007924E−01 A12 1.616913E−02 −2.288862E−01 −1.923592E−01 2.477893E−01 3.207398E−02 2.364562E−01 A14 0.000000E+00 4.526324E−02 3.168858E−02 −3.974356E−02 0.000000E+00 −1.462978E−01 A16 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 2.957069E−02 Eighth Surface Ninth Surface Tenth Surface Eleventh Surface Twelfth Surface Thirteenth Surface k 0.000000E+00 −1.231949E+00 −2.153378E+00 9.892107E+01 −2.615935E+00 −2.620368E+00 A4 1.085136E−03 −1.175223E−01 −5.727628E−02 4.317165E−02 −4.088004E−01 −2.509153E−01 A6 2.897851E−01 2.453127E−01 7.434006E−02 −8.582866E−03 2.154187E−01 1.708782E−01 A8 −6.223866E−01 −3.034900E−01 −8.554123E−02 −2.421852E−02 −8.524043E−02 −7.935889E−02 A10 5.908747E−01 2.222656E−01 4.468345E−02 9.292100E−03 2.743479E−02 2.319218E−02 A12 −2.539663E−01 −8.247856E−02 −1.440364E−02 −1.399569E−03 −6.454056E−03 −4.002166E−03 A14 3.993062E−02 1.176058E−02 2.509092E−03 8.721408E−05 9.224915E−04 3.665499E−04 A16 −7.230835E−04 6.874159E−05 −1.531228E−05 4.236622E−07 −5.661052E−05 −1.366747E−05

The imaging lens in Example 2 satisfies conditional expressions (1) to (13) as shown in Table 6.

FIG. 4 shows spherical aberration (mm), astigmatism (mm), and distortion (%) of the imaging lens in Example 2. As shown in FIG. 4, each aberration is corrected excellently.

Example 3

The basic lens data is shown below in Table 3.

TABLE 3 Example 3 Unit mm f = 2.91 i h = 2.90 Fno = 1.50 TTL = 4.54 ω(°) = 44.3 Surface Data Surface Curvature Surface Refractive Abbe Number i Radius r Distance d Index Nd Number νd (Object) Infinity Infinity 1(Stop) Infinity −0.1800 2* 2.4296 0.5268 1.544 55.57 (νd1) 3* 19.3457 0.0277 4* 2.1251 0.2000 1.661 20.37 (νd2) 5* 1.7175 0.3609 6* 9.3671 0.8054 1.535 55.66 (νd3) 7* −5.4502 0.2226 8* −1.5280 0.2100 1.661 20.37 (νd4) 9* −3.9708 0.0200 10* 1.5406 0.7482 1.535 55.66 (νd5) 11* 4.0459 0.1200 12* 0.7311 0.3968 1.535 55.66 (νd6) 13* 0.6623 0.6000 14  Infinity 0.3000 1.517 64.20 15  Infinity 0.1060 Image Plane Infinity Constituent Lens Data Lens Start Surface Focal Length Composite Focal Length 1 2 5.054 f56 2.870 2 4 −16.834 3 6 6.567 4 8 −3.890 5 10 4.214 6 12 13.021 Aspheric Surface Data Second Surface Third Surface Fourth Surface Fifth Surface Sixth Surface Seventh Surface k 0.000000E+00 −3.000000E+02 −8.771705E+00 −1.193416E+01 0.000000E+00 1.401481E+01 A4 1.308034E−02 −1.677372E−01 −2.065242E−01 8.192967E−01 −6.104116E−02 −9.463175E−01 A6 −3.843767E−02 5.057299E−01 4.821804E−01 −2.183535E−01 −2.424147E−02 3.939789E−02 A8 6.126802E−02 −8.110403E−01 −6.073818E−01 3.810698E−01 2.232673E−02 −4.988421E−02 A10 −7.763567E−02 7.517286E−01 3.698069E−01 −3.494619E−01 −3.806988E−02 −1.353477E−01 A12 6.453798E−02 −3.796968E−01 −6.171676E−02 1.301515E−01 2.540087E−02 2.453992E−01 A14 −2.290033E−02 6.764935E−02 −4.615610E−02 −1.873816E−02 −3.873394E−03 −1.396682E−01 A16 −1.304420E−03 1.304054E−03 0.000000E+00 1.567306E−03 0.000000E+00 2.745815E−02 Eighth Surface Ninth Surface Tenth Surface Eleventh Surface Twelfth Surface Thirteenth Surface k 0.000000E+00 −9.902068E−01 −2.158530E+00 7.230824E−01 −1.938903E+00 −1.728580E+00 A4 7.770752E−02 −2.165223E−01 −9.761274E−02 2.467117E−02 −3.297684E−01 −3.182313E−01 A6 3.719669E−01 4.960984E−01 7.719542E−02 −3.691505E−03 1.673900E−01 2.094083E−01 A8 −7.234703E−01 −5.297540E−01 −4.852361E−02 −1.612803E−02 −6.651520E−02 −9.081110E−02 A10 5.754972E−01 3.088786E−01 1.036096E−02 6.059588E−03 2.123784E−02 2.449373E−02 A12 −2.036982E−01 −9.308772E−02 −6.184045E−04 −8.493768E−04 −4.493806E−03 −3.833641E−03 A14 2.547367E−02 1.161307E−02 −3.413792E−07 3.929675E−05 5.251131E−04 3.166172E−04 A16 1.897606E−04 −1.164551E−04 0.000000E+00 0.000000E+00 −2.562642E−05 −1.069555E−05

The imaging lens in Example 3 satisfies conditional expressions (1) to (13) as shown in Table 6.

FIG. 6 shows spherical aberration (mm), astigmatism (mm), and distortion (%) of the imaging lens in Example 3. As shown in FIG. 6, each aberration is corrected excellently.

Example 4

The basic lens data is shown below in Table 4.

TABLE 4 Example 4 Unit mm f = 3.06 i h = 2.90 Fno = 1.45 TTL = 4.49 ω(°) = 42.2 Surface Data Surface Curvature Surface Refractive Abbe Number i Radius r Distance d Index Nd Number νd (Object) Infinity Infinity 1(Stop) Infinity −0.2400 2* 2.4381 0.5915 1.544 55.57 (νd1) 3* −240.0000 0.0200 4* 1.8542 0.2200 1.661 20.37 (νd2) 5* 1.3559 0.3968 6* 6.4084 0.7202 1.535 55.66 (νd3) 7* −6.9651 0.2234 8* −1.5471 0.2100 1.661 20.37 (νd4) 9* −2.5797 0.0151 10* 1.4418 0.5224 1.535 55.66 (νd5) 11* 3.3429 0.4500 12* 1.0559 0.3784 1.535 55.66 (νd6) 13* 0.7864 0.4000 14  Infinity 0.2100 1.517 64.20 15  Infinity 0.2050 Image Plane Infinity Constituent Lens Data Lens Start Surface Focal Length Composite Focal Length 1 2 4.442 f56 4.958 2 4 −9.264 3 6 6.360 4 8 −6.354 5 10 4.326 6 12 −11.281 Aspheric Surface Data Second Surface Third Surface Fourth Surface Fifth Surface Sixth Surface Seventh Surface k 2.635466E−01 0.000000E+00 −6.377415E+00 −7.147286E+00 −6.000000E+01 1.550000E+01 A4 1.739107E−02 −5.519819E−02 −1.469885E−01 7.871415E−02 −2.085571E−02 −1.458292E−01 A6 −2.396574E−02 2.361691E−01 2.653126E−01 −2.317865E−01 −4.016714E−02 1.513911E−01 A8 1.688381E−02 −4.620359E−01 −4.302119E−01 2.917934E−01 1.695378E−02 −1.591914E−01 A10 −1.012903E−02 4.661897E−01 3.629026E−01 −2.275178E−01 −5.048498E−02 −5.839409E−02 A12 6.626980E−03 −2.442964E−01 −1.386115E−01 9.302490E−02 3.453953E−02 1.897304E−01 A14 −3.226177E−03 5.077895E−02 1.670232E−03 −2.067845E−02 −5.254443E−03 −1.104244E−01 A16 0.000000E+00 −1.093051E−03 4.445452E−03 2.314703E−03 0.000000E+00 2.100000E−02 Eighth Surface Ninth Surface Tenth Surface Eleventh Surface Twelfth Surface Thirteenth Surface k 0.000000E+00 −7.909811E+00 −3.392058E+00 1.561002E+00 −1.671269E+00 −2.440591E+00 A4 6.025961E−02 −1.161559E−01 1.121068E−02 6.678454E−02 −4.819193E−01 −2.769847E−01 A6 4.530902E−01 3.186814E−01 −9.462554E−03 −3.826527E−02 2.878490E−01 1.889927E−01 A8 −8.219108E−01 −3.842492E−01 −1.230093E−02 −1.772804E−02 −1.420469E−01 −9.008448E−02 A10 6.774285E−01 2.619863E−01 1.127138E−03 1.173326E−02 5.607116E−02 2.568900E−02 A12 −2.585490E−01 −8.968538E−02 9.525424E−05 −2.181185E−03 −1.352325E−02 −4.078059E−03 A14 3.636736E−02 1.183101E−02 7.500675E−05 1.359295E−04 1.674573E−03 3.281833E−04 A16 3.141906E−04 3.836870E−05 0.000000E+00 −5.376817E−06 −8.141833E−05 −1.027487E−05

The imaging lens in Example 4 satisfies conditional expressions (1) to (13) as shown in Table 6.

FIG. 8 shows spherical aberration (mm), astigmatism (mm), and distortion (%) of the imaging lens in Example 4. As shown in FIG. 8, each aberration is corrected excellently.

Example 5

The basic lens data is shown below in Table 5.

TABLE 5 Example 5 Unit mm f = 3.31 i h = 2.90 Fno = 1.50 TTL = 4.76 ω(°) = 40.9 Surface Data Surface Curvature Surface Refractive Abbe Number i Radius r Distance d Index Nd Number νd (Object) Infinity Infinity 1(Stop) Infinity −0.3300 2* 2.1406 0.6020 1.544 55.57 (νd1) 3* 23.5863 0.0155 4* 2.0400 0.2000 1.661 20.37 (νd2) 5* 1.5565 0.4607 6* −500.0000 0.7983 1.535 55.66 (νd3) 7* −5.9204 0.1839 8* −1.7552 0.2000 1.661 20.37 (νd4) 9* −3.3110 0.0200 10* 1.7315 0.5954 1.535 55.66 (νd5) 11* 7.6813 0.2981 12* 1.1024 0.4697 1.535 55.66 (νd6) 13* 0.8213 0.5000 14  Infinity 0.2100 1.517 64.20 15  Infinity 0.2771 Image Plane Infinity Constituent Lens Data Lens Start Surface Focal Length Composite Focal Length 1 2 4.287 f56 4.149 2 4 −11.897 3 6 11.196 4 8 −5.958 5 10 4.039 6 12 −14.415 Aspheric Surface Data Second Surface Third Surface Fourth Surface Fifth Surface Sixth Surface Seventh Surface k 6.796622E−01 0.000000E+00 −9.274124E+00 −1.025021E+01 0.000000E+00 1.690000E+01 A4 8.381799E−03 −1.099758E−01 −1.478949E−01 1.256214E−01 −4.102049E−02 −1.221947E−01 A6 −2.544149E−02 3.616598E−01 3.272490E−01 −3.273668E−01 −4.453791E−02 7.805650E−02 A8 4.589624E−02 −5.537791E−01 −4.874659E−01 5.257101E−01 5.828174E−02 −9.097921E−02 A10 −3.557469E−02 4.972717E−01 4.203706E−01 −4.858429E−01 −6.201838E−02 −7.294586E−02 A12 1.302894E−02 −2.392764E−01 −2.025236E−01 2.157886E−01 2.798847E−02 2.010423E−01 A14 0.000000E+00 4.657713E−02 3.331014E−02 −3.471765E−02 −8.906783E−04 −1.257541E−01 A16 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 0.000000E+00 2.564746E−02 Eighth Surface Ninth Surface Tenth Surface Eleventh Surface Twelfth Surface Thirteenth Surface k 0.000000E+00 −2.221992E−01 −2.404203E+00 1.400162E+01 −1.973073E+00 −2.328997E+00 A4 2.443058E−02 −1.197585E−01 −6.687427E−02 4.082776E−02 −4.141993E−01 −2.609294E−01 A6 2.828910E−01 2.564806E−01 8.017958E−02 −1.370791E−02 2.103464E−01 1.718168E−01 A8 −6.150904E−01 −3.131831E−01 −8.725373E−02 −2.100245E−02 −8.188859E−02 −7.881612E−02 A10 5.806350E−01 2.275702E−01 4.463790E−02 9.384027E−03 2.816204E−02 2.320075E−02 A12 −2.482420E−01 −8.327960E−02 −1.358618E−02 −1.748148E−03 −7.313164E−03 −4.006187E−03 A14 3.958836E−02 1.1171424E−02 2.046545E−03 8.202074E−05 1.125840E−03 3.644964E−04 A16 −4.227878E−04 4.023797E−05 −9.495191E−05 6.984873E−06 −7.291178E−05 −1.344956E−05

The imaging lens in Example 5 satisfies conditional expressions (1) to (13) as shown in Table 6.

FIG. 10 shows spherical aberration (mm), astigmatism (mm), and distortion (%) of the imaging lens in Example 5. As shown in FIG. 10, each aberration is corrected excellently.

In table 6, values of conditional expressions (1) to (13) related to the Examples 1 to 5 are shown.

TABLE 6 Conditional Ex- Ex- Ex- Ex- expression ample 1 ample 2 ample 3 ample 4 Example 5 (1) (D3/|f3|) × 100 13.44 9.46 12.26 11.32 7.13 (2) T1/T2 0.05 0.10 0.08 0.05 0.03 (3) (T2/f) × 100 12.71 12.85 12.38 12.88 13.93 (4) f2/f −3.20 −3.63 −5.78 −3.01 −3.60 (5) νd5/νd6 1.00 1.00 1.00 1.00 1.00 (6) (D2/f2) × 100 −2.12 −1.70 −1.19 −2.37 −1.68 (7) (T4/f) × 100 0.93 0.34 0.69 0.49 0.60 (8) f4/f −2.12 −1.91 −1.33 −2.07 −1.80 (9) f5/f 1.40 1.37 1.45 1.40 1.22 (10) f56/f 2.83 1.82 0.98 1.61 1.25 (11) f2/f5 −2.28 −2.65 −4.00 −2.14 −2.95 (12) |r7|/f 0.55 0.55 0.52 0.50 0.53 (13) |f3|/f 1.86 2.06 2.25 2.06 3.39 camera and also high performance thereof.

DESCRIPTION OF REFERENCE NUMERALS

-   ST: aperture stop -   L1: first lens -   L2: second lens -   L3: third lens -   L4: fourth lens -   L5: fifth lens -   L6: sixth lens -   ih: maximum image height -   IR: filter -   IMG: imaging plane 

What is claimed is:
 1. An imaging lens comprising in order from an object side to an image side, a first lens having positive refractive power and a convex surface facing the object side near an optical axis and, a second lens having negative refractive power near the optical axis, a third lens, a fourth lens, a fifth lens having a concave surface facing the image side near the optical axis, and a sixth lens having a concave surface facing the image side near the optical axis, wherein an image-side surface of said sixth lens is formed as an aspheric surface having at least one off-axial pole point, said fifth lens has positive refractive power near the optical axis, an object-side surface of said sixth lens has a convex surface facing the object side near the optical axis, and below conditional expressions (1) and (2) are satisfied: 5.0<(D3/|f3|)×100<14.5  (1) 0.02<T1/T2<0.12  (2) where D3: thickness along the optical axis of the third lens, f3: focal length of the third lens, T1: distance along the optical axis from the image-side surface of the first lens to the object-side surface of the second lens, T2: distance along the optical axis from the image-side surface of the second lens to the object-side surface of the third lens.
 2. An imaging lens comprising in order from an object side to an image side, a first lens having positive refractive power and a convex surface facing the object side near an optical axis, a second lens having negative refractive power near the optical axis, a third lens, a fourth lens, a fifth lens having a concave surface facing the image side near the optical axis, and a sixth lens having a concave surface facing the image side near the optical axis, wherein an image-side surface of said sixth lens is formed as an aspheric surface having at least one off-axial pole point, an image-side surface of said third lens has a convex surface facing the image side near the optical axis, said fourth lens has negative refractive power near the optical axis, and below conditional expressions (2), (3) and (4) are satisfied: 0.02<T1/T2<0.12  (2) 11.5<(T2/f)×100  (3) −8.7<f2/f<−2.0  (4) where T1: distance along the optical axis from the image-side surface of the first lens to the object-side surface of the second lens, T2: distance along the optical axis from the image-side surface of the second lens to the object-side surface of the third lens, f: focal length of the overall optical system of the imaging lens, f2: focal length of the second lens.
 3. The imaging lens according to claim 1, wherein composite focal length of said fifth lens and said sixth lens is positive near the optical axis.
 4. The imaging lens according to claim 1, wherein an image-side surface of said fourth lens has a convex surface facing the image side near the optical axis.
 5. The imaging lens according to claim 2, wherein an object-side surface of said sixth lens has a convex surface facing the object side near the optical axis.
 6. The imaging lens according to claim 1, wherein a below conditional expression (5) is satisfied: 0.5<vd5/vd6<1.5  (5) where vd5: abbe number at d-ray of the fifth lens, vd6: abbe number at d-ray of the sixth lens.
 7. The imaging lens according to claim 2, wherein a below conditional expression (6) is satisfied: −3.60<(D2/f2)×100<−0.55  (6) where D2: thickness along the optical axis of the second lens, f2: focal length of the second lens.
 8. The imaging lens according to claim 2, wherein a below conditional expression (1) is satisfied: 5.0<(D3/|f3|)×100<14.5  (1) where D3: thickness along the optical axis of the third lens, f3: focal length of the third lens.
 9. The imaging lens according to claim 1, wherein a below conditional expression (7) is satisfied: 0.15<(T4/f)×100<1.40  (7) where T4: distance along the optical axis from the image-side surface of the fourth lens to the object-side surface of the fifth lens, f: focal length of the overall optical system of the imaging lens.
 10. The imaging lens according to claim 1, wherein a below conditional expression (8) is satisfied: −3.20<f4/f<−0.65  (8) where f4: focal length of the fourth lens, f: focal length of the overall optical system of the imaging lens.
 11. The imaging lens according to claim 1, wherein a below conditional expression (9) is satisfied: 0.6<f5/f<2.2  (9) where f5: focal length of the fifth lens, f: focal length of the overall optical system of the imaging lens.
 12. The imaging lens according to claim 3, wherein a below conditional expression (10) is satisfied: 0.45<f56/f<4.25  (10) where f56: composite focal length of the fifth lens and the sixth lens, f: focal length of the overall optical system of the imaging lens.
 13. The imaging lens according to claim 1, wherein a below conditional expression (11) is satisfied: −6.0<f2/f5<−1.6  (11) where f2: focal length of the second lens, f5: focal length of the fifth lens.
 14. The imaging lens according to claim 1, wherein a below conditional expression (12) is satisfied: 0.25<|r7|/f<0.85  (12) where r7: paraxial curvature radius of the object-side surface of the fourth lens, f: focal length of the overall optical system of the imaging lens.
 15. The imaging lens according to claim 2, wherein composite focal length of said fifth lens and said sixth lens is positive near the optical axis.
 16. The imaging lens according to claim 2, wherein an image-side surface of said fourth lens has a convex surface facing the image side near the optical axis.
 17. The imaging lens according to claim 2, wherein a below conditional expression (5) is satisfied: 0.5<vd5/vd6<1.5  (5) where vd5: abbe number at d-ray of the fifth lens, vd6: abbe number at d-ray of the sixth lens.
 18. The imaging lens according to claim 2, wherein a below conditional expression (7) is satisfied: 0.15<(T4/f)×100<1.40  (7) where T4: distance along the optical axis from the image-side surface of the fourth lens to the object-side surface of the fifth lens, f: focal length of the overall optical system of the imaging lens.
 19. The imaging lens according to claim 15, wherein a below conditional expression (10) is satisfied: 0.45<f56/f<4.25  (10) where f56: composite focal length of the fifth lens and the sixth lens, f: focal length of the overall optical system of the imaging lens.
 20. The imaging lens according to claim 2, wherein a below conditional expression (11) is satisfied: −6.0<f2/f5<−1.6  (11) where f2: focal length of the second lens, f5: focal length of the fifth lens. 